Conventional indirect indicators of iron status, using serum and red blood cell biomarkers, are confounded by inflammation from common infections in sub-Saharan Africa, a region with a high prevalence of iron deficiency, making the assessment of iron balance and efficiency of iron intervention difficult. A new method allowing accurate measurement of long-term oral iron absorption and allowing the estimation of iron requirements is highly needed. Such a novel method to quantify iron absorption and requirements using isotope dilution measurements in children should be validated in the present prospective observational study by following-up a group of 49 children given a stable iron isotope in an earlier trial. We will request seven blood samples within 2 years (sampling every four months) from the participants which will allow us to measure isotopic dilution for estimating total oral iron absorption over these 24 months.